Solar Cells Having Nanowire Titanium Oxide and/or Silicon Carbide Cores and Graphene Exteriors

ABSTRACT

An apparatus comprising a plurality of solar cells that each comprise a nanowire titanium oxide core having graphene disposed thereon. By one approach this plurality of solar cells can comprise, at least in part, a titanium foil having the plurality of solar cells disposed thereon wherein at least a majority of the solar cells are aligned substantially parallel to one another and substantially perpendicular to the titanium foil. Such a plurality of solar cells can be disposed between a source of light and another modality of solar energy conversion such that both the solar cells and the another modality of solar energy conversion generate electricity using a same source of light.

RELATED APPLICATION(S)

This is a continuation of U.S. patent application Ser. No. 14/420,860,Feb. 10, 2015, entitled Solar Cells Having Nanowire Titanium OxideAnd/Or Silicon Carbide Cores And Graphene Exteriors, which applicationis a 371 of PCT/US13/53939, Filed Aug. 7, 2013, which is acontinuation-in-part of U.S. patent application Ser. No. 13/908,215,Filed on Jun. 3, 2013, now U.S. Pat. No. 8,829,331, which is acontinuation in part of U.S. patent application Ser. No. 13/682,229,filed on Nov. 20, 2012, now U.S. Pat. No. 8,586,999, which claims thebenefit of U.S. Provisional Application No. 61/681,873, filed Aug. 10,2012, which is incorporated by reference in its entirety herein.

TECHNICAL FIELD

This invention relates generally to the generation of electricity usingsolar energy.

BACKGROUND

Various modalities of solar energy conversion are known in the art. Itis known, for example, to employ photovoltaic junctions in favor ofconverting sunlight directly into electricity. It is also known toconvert sunlight into heat and to then utilize that heat to generateelectricity.

That such approaches are known, however, is not a panacea in and ofitself. Various implementation and operating problems arise with eachsuch modality of solar energy conversion. Conversion efficiency, forexample, varies amongst different modalities with many positedmodalities performing quite poorly and none achieving high conversionefficiency. Those relative conversion efficiencies, in turn, quitedirectly impact the return on investment an enterprise can expect toreceive for pursuing a given solar energy conversion system.

Dispatchability is another such example. Dispatchability refers to anability to contribute electricity to a distribution grid at times ofneed. Since solar energy conversion requires the presence of the sun,converting solar energy into electricity cannot occur at night in theabsence of an energy-storage mechanism. Energy storage can give rise tonew concerns, however. Using batteries on a large scale for energystorage, for example, is relatively expensive in a variety of ways.

BRIEF DESCRIPTION OF THE DRAWINGS

The above needs are at least partially met through an apparatuspertaining to solar cells having silicon carbide and/or nanowiretitanium oxide cores and graphene exteriors and the co-generationconversion of light into electricity described in the following detaileddescription, particularly when studied in conjunction with the drawings,wherein:

FIG. 1 comprises a side-elevational, sectioned schematic view asconfigured in accordance with various embodiments of the invention;

FIG. 2 comprises a side-elevational schematic view as configured inaccordance with various embodiments of the invention;

FIG. 3 comprises a top plan view as configured in accordance withvarious embodiments of the invention;

FIG. 4 comprises a perspective schematic view as configured inaccordance with various embodiments of the invention;

FIG. 5 comprises a perspective view as configured in accordance withvarious embodiments of the invention;

FIG. 6 comprises a top plan view as configured in accordance withvarious embodiments of the invention

FIG. 7 comprises a top plan view as configured in accordance withvarious embodiments of the invention;

FIG. 8 comprises a side-elevational schematic view as configured inaccordance with various embodiments of the invention

FIG. 9 comprises a side-elevational schematic view as configured inaccordance with various embodiments of the invention;

FIG. 10 comprises a perspective schematic view as configured inaccordance with various embodiments of the invention;

FIG. 11 comprises a side-elevational schematic view as configured inaccordance with various embodiments of the invention; and

FIG. 12 comprises a block diagram view as configured accordance withvarious embodiments of the invention.

Elements in the figures are illustrated for simplicity and clarity andhave not necessarily been drawn to scale. For example, the dimensionsand/or relative positioning of some of the elements in the figures maybe exaggerated relative to other elements to help to improveunderstanding of various embodiments of the present invention. Also,common but well-understood elements that are useful or necessary in acommercially feasible embodiment are often not depicted in order tofacilitate a less obstructed view of these various embodiments of thepresent invention. Certain actions and/or steps may be described ordepicted in a particular order of occurrence while those skilled in theart will understand that such specificity with respect to sequence isnot actually required. The terms and expressions used herein have theordinary technical meaning as is accorded to such terms and expressionsby persons skilled in the technical field as set forth above exceptwhere different specific meanings have otherwise been set forth herein.

DETAILED DESCRIPTION

Generally speaking, pursuant to these various embodiments, a pluralityof solar cells can each comprise a nanowire titanium oxide core havinggraphene disposed thereon. By one approach this plurality of solar cellscan comprise, at least in part, a titanium foil having the plurality ofsolar cells disposed thereon wherein at least a majority of the solarcells are aligned substantially parallel to one another andsubstantially perpendicular to the titanium foil.

A corresponding apparatus can comprise a first modality of solar energyconversion that is disposed between a source of light and a secondmodality of solar energy conversion. So configured, both the firstmodality of solar energy conversion and the second modality of solarenergy conversion can independently generate electricity using a samesource of light. (As used herein, this reference to a “second” modalitywill be understood to not merely refer to a numerically additional orsupplemental modality but will instead be understood to refer to anotherand different-in-kind modality as compared to the first modality ofsolar energy conversion.)

By one approach the first modality of solar energy conversion comprisesa graphene-based modality of solar energy conversion such as theaforementioned solar cells that each comprise a nanowire titanium oxidecore having graphene disposed thereon. The graphene-based modality ofsolar energy conversion can comprise a photovoltaic transducer thatdirectly converts light into electricity. By one approach the secondmodality of solar energy conversion serves to convert heat intoelectricity. In such a case, the graphene-based modality of solar energyconversion can be disposed directly on a heat-absorbing surface thatcomprises a part of the second modality of solar energy conversion. Thatheat-absorbing surface, for example, can comprise a fluid-carryingconduit.

Fluids employed in such a second modality of solar energy conversion canbe in excess of 350 degrees Centigrade. Such temperatures, of course,are quite inhospitable to the functioning of many ordinary photovoltaicdevices that employ silicon or gallium arsenide. Graphene, however, iscapable of operating in a satisfactory manner at such temperatures.Accordingly, a graphene-based photovoltaic junction can be expected tomaintain photovoltaic functionality at a useful level of energyconversion notwithstanding the high temperatures of the adjacent secondmodality of solar energy conversion.

Accordingly, so configured, such a system comprises a cogenerationsystem for converting light into electricity. Depending upon the designemployed the relative conversion efficiency of each modality can berelatively similar. By one approach, electricity generated by thegraphene-based modality of solar energy conversion can be provided inreal time to a distribution grid. Heat generated by the second modalityof solar energy conversion, however, can be utilized immediately togenerate electricity or can be stored and utilized later (for example,during a late afternoon period of peak loading and/or during the eveninghours) to generate electricity to contribute to the distribution grid.

Such a co-generation facility offers other advantages beyond theimproved dispatchability capability noted above. For example, therelative cost of such a co-generation facility need not be appreciablymore than the cost of building and operating the second modality ofsolar energy conversion as an isolated system. To some very real extent,the present teachings permit leveraging the availability of such asystem by greatly increasing the power output of the overall systemwhile greatly enhancing the ability to time shift the distribution ofthe generated electricity. Accordingly, the present teachings offer ahighly flexible approach to generating and distributing electricity.

Graphene is a substance made of pure carbon, with atoms arranged in aregular hexagonal pattern similar to graphite, but in a one-atom thicksheet (hence leading to the oft-used graphene descriptor“two-dimensional”). Graphene is the basic structural element of manycarbon allotropes including graphite, charcoal, carbon nanotubes, andfullerenes. Graphene has a number of interesting optical, electrical,and thermal properties and has been the subject of much recent research.For example, the 2010 Nobel Prize in Physics went to researchers workingon “groundbreaking” experiments involving graphene.

A single layer of graphene is nearly transparent. That said, however, astack of forty-three graphene layers has an opacity of about ninety-ninepercent and essentially completely absorbs light throughout the visibleand near infrared regions of the solar spectrum notwithstanding having afilm thickness of less than fifteen nanometers.

Referring now to the drawings, FIG. 1 depicts an object 100 having acore 101 that consists essentially of a wide band-gap material. Thisreference to a wide band-gap material will be understood to refer to amaterial having a valence band and a conduction band that differ by atleast two electron volts. By one approach this core 101 can comprisetitanium oxide. The reference to the core 101 consisting essentially ofthis material will be understood to refer to a core 101 that is largelypure in these regards but which can include trace impurities (and whichcan also include, for example, purposefully-introduced n-type or p-typedopants designed to elicit specific electrical performance).

In FIG. 1 the core 101 comprises a sphere. As will be shown below thecore 101 can in fact assume any of a wide variety of regular andirregular forms. That said, by one approach the core 101 has at leastone nanoscale bisectional dimension that does not exceed, for example,one hundred nanometers. This requirement does not specify that alldimensions of the core 100 are so limited, however. When the core 101comprises, for example, a nanowire the core 101 might have alongitudinal length in excess of many hundreds of nanometers. (It is notnecessary that the referred-to bisector divide the core 101 into twoequal halves. Instead, as used herein a reference dimension is a“bisector” if the corresponding line passes through opposing points onthe periphery and includes a geometric center of the core 101 as per acorresponding frame of reference. For example, when the core 1011comprises a cylinder, a lateral cross-sectional diameter of thatcylinder can serve as a bisector for these purposes.)

The depicted object 100 also has a shell 102 consisting essentially ofgraphene conformally disposed about at least a substantial portion ofthe core. As used herein this reference to “substantial” will beunderstood to refer to an amount in excess of fifty percent.

The thickness of this graphene shell 102 can vary with the applicationsetting. For many purposes, however, the thickness can range from aboutone layer to about forty-three layers or so. Accordingly, for manypurposes the thickness of the shell 102 is very thin. Generallyspeaking, when employed in a photovoltaic setting, the number of layersneed be no more than are required to achieve a particular amount oflight absorption (for example, forty-three layers of graphene willabsorb nearly all incident light). It will be presumed herein that theshell 102 has an essentially uniform thickness for a given object 100but these teachings will accommodate variations in these regards ifdesired and/or as appropriate to the needs of a given applicationsetting. For example, in some application settings it can be useful ifthe shell 102 has no more than about ten layers of graphene

Generally speaking, electron/hole recombination rates are dependent on avariety of factors. In order to achieve a high efficiency solar cell,the material selected for the core 101 can be chosen to optimize chargeseparation in preference to electron/hole recombination.

By one approach, the shell 102 is conformally applied to the core 101 bycontacting a reactant at elevated temperatures with nanopowders ofdifferent compounds such as borides, carbides, nitrides, oxides, andsulfides. Typical examples of such compounds are boron carbide, siliconcarbide, silicon nitride, silicon oxide, titanium oxide, zinc oxide,iron sulfide, and so forth.

More particularly, by one approach the corresponding synthesis carriesout the reaction(s) using size and shape-selected nanopowders of theselected material for the core 101. Per the reaction, hydrocarbonmolecules decompose on the surfaces of the nanopowders to form thegraphene layer(s) that comprises the shell 102. In particular, thenanomaterials cited above act as catalysts for the reactions to formgraphene and result in highly unusual core/shell nanostructures in whichthe shell 102 is graphene and the cores 101 are composed ofnanoparticles of the particular starting material. Additions of dopantsduring or after synthesis allows their incorporation into the shell orfunctionalization of the shell to produce desired dopant levels.

By one approach the reactions to form the graphene occur with firstorder kinetics at temperatures that are typically about 1000 degreesCentigrade lower in the presence than in the absence of the nanopowders.At least one reason for this dramatic lowering in reaction temperatureis believed to be the ubiquitous presence of defects of various kinds onthe surfaces of the nanomaterials. Because of the large surface area ofthe nanomaterials (relatively speaking) and the lowering of theactivation energies of the reactions due to the presence of detects, thereaction kinetics are rapid at modest reaction temperatures.

The reaction rates are largely determined by temperature andtemperature, of course, can be carefully controlled. Shell 102 thickness(i.e., the number of graphene layers), however, is largely controlled bythe length of time the reaction is allowed to proceed at a controlledtemperature. A layer-by-layer growth mechanism is therefore inherent inthe presently-disclosed synthesis method making this a very powerful wayof precisely controlling a key parameter that can serve as an importantdeterminant of the ultimate performance of the material in a solar celldevice.

This extremely versatile aspect of the described one-step synthesismethod therefore allows one to adjust, in addition to core size andcomposition, the shell thickness parameter so as to be able to optimizethe graphene photophysics. As alluded to above, the photophysics ofgraphene (involving excited-state transition lifetimes andcharge-transport kinetics) can be highly useful to achieve ahighly-efficient and commercially-viable solar cell. For example, a filmconsisting of 20 nanoparticles (each sized from around ten to eightynanometers) that each have a monolayer graphene shell 102 would have athickness of about 200-1600 nanometers and would absorb aroundninety-five percent of the incident light. Such a very thin film solarcell would be expected to operate at considerably higher efficiencieswhile being considerably less costly than prior art solar cells.

The examples above presumed to encapsulate individual small particleswith a graphene shell. These teachings will accommodate otherapproaches, however. By one approach, for example, the core 101 cancomprise a plurality of nanoparticles 201 that are annealed to oneanother as illustrated in FIG. 2. In this simple example the resultantnanowire of the wide band-gap material comprises threesequentially-connected nanoparticles 201. It will be understood, ofcourse, that such a nanowire can comprise a considerably larger numberof such nanoparticles 201. Accordingly, although the lateral crosssection of such a nanowire may remain within the aforementioned onehundred nanometer bisectional dimensional limitation the length of thenanowire can be of essentially any dimension of choice. By one approachthe shells are applied subsequent to the annealing step.

By way of a more specific example (but without intending any specificlimitations by way of these corresponding details), silicon carbidenanowires can be synthesized in a variety of ways. For example, chemicalvapor deposition procedures can serve in these regards. As anotherexample, silicon carbide nanoparticles can be heated to 2000K in thepresence of a catalyst such as iron powder.

The graphene shells 102 surrounding the nanowires can in turn besynthesized to a predetermined thicknesses by reaction of the nanowireswith carbonaceous substances at elevated temperatures as already statedor by heating the nanowires in vacuum to 1500K or in one bar of argon to2000K. One straightforward and economically attractive synthesisprocedure for objects 100 having a silicon carbide nanowire core 101 anda graphene shell 102 would comprise heating commercial silicon carbidepowders to 2000K in argon to grow the nanowires, following which thecatalyst is removed and the resultant material then reheated to 2000K inargon to form epitaxial graphene layers to a predetermined thickness anddoping level

It may be noted and emphasized that these teachings will accommodateusing cores 101 having any of a wide variety of forms. With reference toFIG. 3, for example, the cores 101 can comprise nanowires 300. Asillustrated in FIG. 4, a graphene shell 102 of a desired thickness canthen be readily conformally applied to such a core 101. By way offurther illustrative examples, FIG. 5 illustrates a core 101 comprisinga nanotube 500, FIG. 6 illustrates cores 101 comprising nanofibrils 600,and FIG. 7 illustrates a core 101 comprising a nanofabric 700. Again,these examples are intended to represent a non-exhaustive listing ofpossibilities in these regards.

Generally speaking, a viable solar cell will comprise a plurality ofsuch objects 100 and hence will comprise a plurality of cores 101 thateach have a shell 102 disposed thereabout. By one useful approach, whenthe cores 101 comprise generally longitudinal members (such asnanowires, nanorods, and/or nanotubes), the plurality of cores 101 canbe disposed with their longitudinal axes oriented at least substantiallyparallel to one another. In particular, by one approach theselongitudinal axes are also oriented substantially coaxial and parallelto anticipated light beams.

FIG. 8 presents one illustrative example in these regards. In thisillustrative example a solar cell 800 has a first electrical pathway 801and an opposing second electrical pathway 802 (the latter beingtransparent or at least translucent to permit light 804 to passtherethrough). These two electrical pathways 801 and 802 in turn coupleto an electrical load 803 of choice such as but not limited to anelectricity-distribution grid (via, for example, one or moretransformers, regulators, and so forth as desired).

In this example, each of the cores 101 physically and electricallyconnects to the first electrical pathway 801 but not to the secondelectrical pathway 802. By one approach, the longitudinal axes of thesecores 101 can range from about two hundred nanometers to no more thanabout five thousand nanometers.

By one approach, the nanowires that comprise the cores 101 in thisexample essentially consist of silicon carbide and have a length of fromabout three hundred to about five thousand nanometers andcross-sectional diameters of from about ten to about eighty nanometerscoaxially. These cores 101 are surrounded by anywhere from a monolayerforty-three or so layers of graphene that essentially constitute thegraphene shells 102.

These aligned silicon carbide cores 101 can be grown prior to growingthe shells 102 by reacting a dilute mixture of methane in hydrogen witha silicon substrate carrying a catalyst to enable a vapor-liquid-solidgrowth mechanism. By one approach the catalyst facilitates the growth ofaligned silicon carbide nanowires. A catalyst that forms agallium/iron/silicon alloy, for example, allows the reaction with thecarbon in the methane to occur in such a way as to lead to the growth ofaligned silicon carbide nanowires.

The aligned silicon carbide nanowires can be grown the desired length byallowing the reaction to occur for a corresponding length of time at atemperature of about 1300 to about 1600 K. When the desired nanowirelength has been reached, the gas mixture flowing over the nanowires ischanged to pure methane to thereby create conditions for growing thegraphene shells 102. Again, the number of layers in the shells 102 iscontrolled by the length of time growth is allowed to proceed at a giventemperature.

The bottoms of the resulting nanowire nanostructures are attached to thefirst electrical pathway 801 that comprises, for example, silicon orsome other conducting base, thus electrically connecting all of thecores 101. The tops of the nanowires carry graphene layers that are inelectrical contact with all of the coaxial graphene shells 102surrounding the nanowire cores 101. The growth process thusautomatically creates a solar cell nanostructure that provides separateelectrical connections to cores 101 and shells 102.

By one approach, dark current and majority carrier transport can beminimized by preventing contact of the coaxial graphene shells 102 withthe silicon base and with the silicon carbide nanowire cores. This canbe accomplished, for example, by chemical vapor deposition of amonolayer or multilayers of an insulator such as silicon dioxide priorto depositing the graphene shells 102.

Solar energy conversion efficiency associated with such a configurationis strongly enhanced in the nanostructured radial coaxial shell/corenanowire configuration over conventional planar solar cellconfigurations in part because electron diffusion lengths are very muchshorter. This is particularly the case with regard to the thin grapheneshells 102.

In addition, it may be observed that such a solar cell functions byvirtue of the internal photoemission of energetic electrons emanatingfrom the graphene layers and that are injected into the conduction bandof the silicon carbide cores loll upon irradiation by light. The tens ofmilliamperes per square centimeter of photocurrent produced in this wayis expected to have energies of about one electron volt under standardsunlight intensities. The stated cell performance is due in part to thepotential barrier created when graphene forms a junction with siliconcarbide. The magnitude of this potential barrier is estimated to besomewhat less than one electron volt based on calculations using theAnderson approximation which employs known values of quantities such asthe work functions, electron affinities and electrical conductivities ofgraphene and silicon carbide

The described graphene/silicon carbide junction is also noteworthy inthat the junction joins graphene (a zero band-gap material in whichelectrons behave as if they were massless fermions obeying Dirac wavefunctions according to the theory of quantum electrodynamics) to a wideband-gap semiconductor, silicon carbide, in which electrons obey thedictates of quantum mechanics. One of the functions of this junction,whose width is less than one nanometer, is to effectively transform“Dirac” electrons into “Schroedinger” electrons during the process ofphotoemission while concurrently minimizing the recombination ofelectrons with holes.

Generally speaking, restructuring of the atoms constituting such ajunction has been shown to occur during junction synthesis. Thisrestructuring still further enhances the interfacial area thusfacilitating the complex functions of the junction. It is known, forexample, that the position of the Fermi level of graphene changes at theheterojunction with a wide band-gap semiconductor. This exceptionalcircumstance does not occur with other metal/semiconductorheterojunctions at least in part because unlike the very large number ofquantum states found with ordinary metals, graphene has only a smallnumber of states near the Fermi level. The unique quantum physics of thegraphene/semiconductor heterojunction is expected to benefit thecharacteristics of quantum structured solar cells such as their darkcurrent behavior.

A useful attribute of a graphene/silicon carbide solar cell is itstemperature stability. Both graphene and silicon carbide arehigh-temperature materials in that they have very high melting pointsand are thermodynamically stable. Diffusional processes of dopants inthese materials are very slow at temperatures up to about 1000K thushelping to preserve the rectifying properties of the correspondingSchottky barrier up to temperatures of at least 600K.

The graphene/wide band-gap semiconductor materials all share theseattributes with graphene/silicon carbide being an example. Thegraphene/wide band-gap materials are therefore uniquely capable offunctioning as high-temperature solar cells in a co-generationapplication setting.

As another example in these regards (and still referring to FIG. 8),these teachings will accommodate providing a titanium foil and using ahydrothermal synthesis process to form a plurality of substantiallyaligned titanium oxide nanowires to serve as the nanowire cores 101.Again, this plurality of nanowire cores 101 are disposed substantiallyperpendicular to the titanium foil. (As used in these regards, the word“substantially” will be understood to mean within twenty-five degrees.)(As it is known in the art to form titanium oxide nanowires on atitanium foil using a hydrothermal synthesis process that exposestitanium foils to aqueous solutions consisting of mixtures of alkalinehydroxides in pressure vessels at elevated temperatures, for the sake ofbrevity no further elaboration is provided here in these particularregards aside from noting that, upon growing the desired alignednanowires, these teachings will accommodate activating correspondingpressure relief values to thereby permit removing foil sections thathave the desired aligned nanowires and replacing such foil sections withfresh foil sections to permit the process to be repeated upon closingthe relevant pressure valves.)

By one approach such titanium oxide nanowires 101 can be doped (eithern-type or p-type as desired) using a post-growth heat treatment processin conjunction with one or more predetermined (i.e., intended ratherthan accidental or incidental) dopants of choice. As one illustrativeexample in these regards, but without intending to suggest anyparticular limitations in these regards, p-type doping can beaccomplished by heating the aforementioned titanium oxide nanowires 101in a vacuum or by treatment with hydrogen gas (or another reactant ofchoice) at temperatures of 600 to 900 degrees Centrigrade forcorresponding appropriate lengths of time and at particular pressures(to achieve a particular doping level).

Such an approach can serve to introduce oxygen vacuities in the titaniumoxide lattice to thereby effectively create titanium in a trivalentstate. Such doping can serve to reduce the average valency of thetitanium oxide nanowires 101 to thereby increase the electricalconductivity thereof in a controlled way. Such an approach to p-typedoping can help to optimize the characteristics of the graphene-titaniumoxide junction in order to yield a solar cell that displays maximumefficiency with respect to converting light to electricity.

After such doping, the aligned titanium oxide nanowires 101 can beexposed to a hydrocarbon gas such as methane at temperatures of 900 to1000 degrees Centrigrade for appropriate lengths of time to accomplishthe desired chemical vapor deposition of graphene in the spaces betweenthe wires as well as on the top surfaces of the nanowires 101themselves. By one approach this graphene shell 102 is only lightlyapplied and comprises, for example, no more than about ten layers inthickness.

By one illustrative approach, and referring to FIG. 9, such solar cells800 can be employed in conjunction with one or more Fresnel lenses 901including linear Fresnel lens installations. By one approach, and asillustrated, it would even be possible to dispose the solar cells 800directly on the lens 901 itself.

By way of another illustration and referring now to FIGS. 10 and 11,solar cells employing the present teachings can be applied incombination with a second modality of solar energy conversion such as asolar-heat based system 1000 having fluid-carrying tubes 1002 thatabsorb heat from sunlight 804 reflected from a corresponding parabolictrough solar collector 1001. In particular, and as illustrated inparticular in FIG. 11, solar cells 800 as described herein can beexternally disposed on at least a portion of the liquid-carrying conduit1002 itself (such as on a portion of the conduit 1002 that receives thereflect light from the parabolic trough solar collector 1001). When thesolar cells include a titanium foil as described above, the solar cells800 can be disposed on the liquid-carrying conduit 1002 by physicallyattaching the titanium foil itself to the surface of the fluid-carryingsolar collector 1001

FIG. 12 provides another illustrative example in these regards. In thisexample a co-generation facility 1200 employs a tower 1201 having asolar collector section 1202. Light 804 (from a star such as the Earth'ssun 1203) may be received at the solar collector section 1202 directlyfrom the light source but is also received, in a relatively greaterquantity, as reflected from (typically) a plurality of solar reflectors1204.

In this example the solar collector section 1202 includes one or morefluid-carrying conduits (for example, as shown in FIGS. 10 and 11) thatcarry a fluid, such as a molten salt, that is heated by the receivedlight. As described above, grapheme-based photovoltaic transducers areagain directly disposed on these conduits and serve to directly convertreceived sunlight into electricity. Again, when the graphene-basedphotovoltaic transducers include a titanium foil as described above, thegraphene-based photovoltaic transducers can be disposed on theseconduits by physically attaching the titanium foil itself to the surfaceof these conduits.

This electricity can be directed via one or more conductors 1205 to adistribution grid 1210 or, if desired, to an electricity-storagefacility such as a bank of batteries.

The aforementioned hot fluid (which can exceed four hundred degreesCentigrade at operating temperatures) flows in this system to a storagetank 1206 where the heat can be banked until needed. When needed, thatheat drives a steam generator 1207 that provides steam to a turbinegenerator 1208 that generates electricity to be provided to thedistribution grid 1210. Cooled/cooling salt in such a system can bepassed to and stored in a cold salt storage tank until needed.

So configured, direct conversion of photons to electricity is providedby the solar cells 800 while the associated heat is converted toelectricity via, for example, conventional electromagnetic inductiontechnology. Such a co-generation solar-based system can potentiallyconvert around fifty percent of the solar flux energy content atmoderate temperatures to electricity with photovoltaics andelectromagnetic induction making roughly equal contributions.

The efficiency of such nanowire core solar cells depends not only on thenature of the junction between them as explained above but also on theability of light to be absorbed by the nanostructured graphene shells inthe first instance. In particular, in two out of three dimensions, suchnanowires are small as compared to the wavelength of solar radiation. Infact, the absorption of light appears to be enhanced by about a factorof two for such nanowires as compared to bulk material. At least part ofthe reason for this appears to be that electromagnetic radiation entersor is transmitted by the aligned nanowires along their entire length.Thus, the large surface-to-volume ratio (compared to bulk materials)contributes significantly to the quantity of absorbed light.

As a result, radiation is absorbed very efficiently by such grapheneshell/silicon-carbide-or-titanium-oxide nanowires. Furthermore, thelight is distributed over a large graphene surface area because thegraphene shells 102 surround each nanowire in such an embodiment. It islikely therefore that much higher radiation intensities can be toleratedby such nanostructured cells.

Each one of the vast numbers of such nanowires present in such aphotovoltaic device comprises a unique structure that individually actsas an effective nantenna and optical rectifier. Calculations forcylindrical nanowires of radius twenty nanometers show that total lightabsorption occurs for nanowire lengths of about four hundred nanometers(which is, in turn, in the range of the wavelengths of visible light).Each nanowire can therefore be seen to simultaneously possess both thelight (electromagnetic wave) gathering power of an antenna and therectifying properties of a photovoltaic device.

The opacity of graphene (which is generally independent of thewavelength of light), combined with its use as a nantenna shellsurrounding the nanowires, helps to ensure maximal harvesting of theincident sunlight. Consequently, the high solar collection efficiency ofthese graphene-based nantennas coupled with the highly efficientrectification properties of thegraphene/silicon-carbide-or-titanium-oxide junctions combine to create anew and highly efficient class of solar cells.

In addition to such clear benefits as an enabler of efficient andinexpensive solar cells, these teachings can also be readily extended toother areas of interest. For example, these teachings can be readilyleveraged in favor of a new class of biosensors: color sensitive retinalimplants. Just as cochlear implants stimulate auditory nerve cells inthe cochlea of the ear, so retinal implants stimulate cells in theretina of the eye that connect to the optic nerve. The search for suchvisual aids has been underway for a long time with the ultimate goal ofproviding color vision to the visually impaired. The ability to optimizethe electrical response to light wavelength by controlling the length ofthese nanowires make these nanostructures uniquely suited to colordiscriminative retinal implant applications. Furthermore, because thesebiosensors are photosensors as well as photovoltaics, the incoming lightitself can source the energy that powers their operation thuspotentially obviating the need for batteries.

It will be appreciated that although the aforementioned first modalityof solar energy conversion generates electricity photovoltaically whilethe second modality of solar energy conversion generates electricitythermally, the first modality will also typically produce heat as abyproduct of photovoltaic conversion. This heat can in fact augment thetotal electricity generated thermally by the second modality of solarenergy conversion.

Those skilled in the art will recognize that a wide variety ofmodifications, alterations, and combinations can be made with respect tothe above described embodiments without departing from the scope of theinvention, and that such modifications, alterations, and combinationsare to be viewed as being within the ambit of the inventive concept.

We claim:
 1. An apparatus comprising: a core consisting essentially ofwide band-gap material, wherein the core comprises a longitudinalmember; a shell consisting essentially of graphene conformally disposedabout at least a substantial portion of the core, the shell beingpartially transparent to sunlight.
 2. The apparatus of claim 1 whereinthe shell comprises at least a single layer of graphene.
 3. Theapparatus of claim 1 wherein the shell comprises no more than about tenlayers of graphene.
 4. The apparatus of claim 1 wherein the longitudinalmember comprises a nanowire.
 5. The apparatus of claim 4 wherein thenanowire consists of a cylindrically-shaped member.
 6. The apparatus ofclaim 4 wherein the length of the nanowire is more than twenty timesgreater than a widest lateral cross-section thereof.
 7. The apparatus ofclaim 6 wherein the lateral cross section is at least twenty nanometers.8. The apparatus of claim 1 wherein the wide band-gap material comprisesat least one of: boron; iron; titanium; zinc; carbon; and silicon. 9.The apparatus of claim 8 wherein the wide band-gap material furthercomprises at least one of: a boride; a carbide; a nitride; an oxide; anda sulfide.
 10. The apparatus of claim 1 wherein the wide band-gapmaterial comprises at least one of: a boride; a carbide; a nitride; anoxide; and a sulfide.
 11. The apparatus of claim 1 wherein the apparatuscomprises a plurality of the cores that each have one of the grapheneshells disposed thereabout.
 12. The apparatus of claim 10 wherein theplurality of cores are disposed with their longitudinal axes orientedsubstantially parallel to one another.
 13. The apparatus of claim 11wherein the longitudinal member has a length of at least 400 nanometers.14. The apparatus of claim 12 wherein each of the cores comprises ananowire having a radius of at least 20 nanometers.
 15. The apparatus ofclaim 1 wherein the connection between a pathway connecting the shelland the core comprises a photovoltaic junction.
 16. The apparatus ofclaim 1 wherein the core and the shell can include a dopant.
 17. Aphotovoltaic transducer comprising: a core consisting essentially of awide band-gap material, wherein the core comprises a nanowire; a shellconsisting essentially of graphene conformally disposed about at least asubstantial portion of the nanowire, the shell being partiallytransparent to sunlight; wherein sunlight entering the nanowire and thatis transmitted along the nanowire is at least mostly absorbed by thegraphene notwithstanding the partial transparency of the shell.
 18. Thephotovoltaic transducer of claim 17 wherein the graphene comprises atleast a single monolayer of graphene.
 19. The photovoltaic transducer ofclaim 18 wherein all of the sunlight that enters the nanowire and thatis transmitted along the length of the nanowire is substantiallyabsorbed by the graphene shell.
 20. The photovoltaic transducer of claim17 further comprising: a plurality of the nanowires disposed on a sharedsubstrate, wherein each of the nanowires has one of the graphene shellsconformally disposed thereabout.
 21. The photovoltaic transducer ofclaim 20 wherein each of the nanowires comprising the plurality of thenanowires are disposed substantially perpendicular to the sharedsubstrate.